Steam Valve back pressure 1

[Bill3752]

We recently sized a relief valve for steam service. The vendor (Crosby) rep recommended an HSJ conventional valve (2 1/2 K 4). I did some quick calcs and critical flow is computed, even with discharge to atmosphere; with a small bit of tailpipe and an elbow, the dP is much greater than 10%.
Two questions:

1. Does it make sense for a manufacturer to design a valve, such that there is choked flow at the discharge?

2. I was told, but cannot confirm, that the dP must be limited to 3% for an ASME Code Section I valve. It that is the case, I don't believe we could achieve this without a very large RV outlet flange.

I suspect I am missing something here. Any thoughts?

 

[insult2injury]

1. This is a common condition.

2. 3% is the allowable accumulation for a section I valve.

I2I

 

[JAlton]

ASME Section VIII, Pressure Vessels, Non-Mandatory Appendix M, recommends no more than 3% Pressure Drop in the inlet piping. Overpressure is 3% for an ASME Section I, Power Boiler, Safety Valve such as the Crosby HSJ. In other words, the HSJ is designed to reach full lift and flow capacity by 103% of Set Pressure. However, the actual Boiler is allowed 6% Accumulation (similar to Overpressure). This is for multiple Valves on the Boiler.

JAC